Spent an hour or two trying to track down why mu4e was locking up Emacs for 20 seconds or so while updating the mail index in the background. This was occurring during final step of the mu4e update - updating contacts.

It seems that the cause was a large amount of spam I've been getting to a Mailman instance I manage. Each message was coming in with a unique sender, made up of Chinese characters. Indexing was comping back with something like [mu4e] Contacts received: 35000. The *mu4e-log* buffer, which shows when mu4e-debug is set, suggests that the slowness is in communicating these contacts between mu and Emacs, not mu itself. These contacts are used for auto-completion.

Anyhow, clearing out the spam fixed the problem. There may be a performance issue with mu4e and contacts with Chinese characters, or it may simply be loading such a high volume of contacts. Maybe reloading contacts after each index isn't such a good idea also. Anyhow, it's working well for me now.